The present invention relates to a method for the preparation of cis isomers of ethylenically unsaturated compounds or, more particularly, to a method for the selective preparation of cis isomers of ethylenically unsaturated organic compounds by the partial hydrogenation of corresponding acetylenically unsaturated compounds.
As is well known, various kinds of cis isomers of ethylenically unsaturated compounds are very important in the fields of perfumery, agricultural chemicals, medicines and the like and an important method for the preparation thereof is the partial hydrogenation of the triple bond in acetylenically unsaturated compounds into double bond.
Such a selective partial hydrogenation can be performed only by use of a suitable catalyst and one of the most widely used catalysts for the purpose is the so-called Lindlar catalyst which is a palladium catalyst borne on a calcium carbonate carrier and partially poisoned or inactivated with lead. Further, an improvement in the selectivity of the hydrogenation with the Lindlar catalyst up to a practical level has been proposed in which the Lindlar catalyst is used in combination with an amine compound such as quinolines, hexamethylenetetramine, a lower alkylamine and the like (see, for example, Japanese Patent Publications Nos. 35-12762, 36-23317 and 38-64875). One of the problems in the combined use of such amine compounds is the contamination of the product compound with the amine compound used in the hydrogenation reaction so that the use of an amine compound with unpleasant odor is absolutely unsuitable, especially, when the cis isomer as the hydrogenation product is to be directed to the application in perfumery in which any smallest amount of such a contamination is fatal for the quality of the product.
Another problem inherent to the Lindlar catalyst is in the separation and recovery of the catalyst after completion of the hydrogenation reaction. Since the Lindlar catalyst is borne on a calcium carbonate carrier, the catalyst is likely comminuted in the course of the reaction into very finely divided powder so that the separation of the catalyst from the reaction mixture can be performed only with an extreme difficulty. Distillation of the reaction mixture containing the catalyst unremoved in a fine powdered form, even if the amount is very small, is sometimes undesirable, especially, when the product is directed to the perfumery use because of the possible deleterious effects on the quality of the perfume product.
Thus, it has long been desired to develop a novel catalyst for the partial hydrogenation of a disubstituted acetylenic compound into corresponding ethylenically unsaturated compound, in particular, in the form of the cis isomer, which can give high selectivity and reactivity in the reaction and high quality of the products suitable for use even in perfumery as well as easiness in handling thereof.